Previously, there is known an HUD device that includes a display member, which transmits an external image located at an outside of a cabin of a vehicle, such as an automobile, in a manner that enables visual recognition of the external image by an occupant located in an inside of the cabin, and a projector, which projects an optical display image onto the display member.
In the HUD device recited in the Patent Literature 1, a front reflection surface, which forms a virtual image by reflecting an optical display image at an interior side of a cabin, and a back reflection surface, which forms a virtual image by reflecting the optical display image on an exterior side of the cabin, are formed in a display member. A cross section of the display member is configured such that an optical axis of the image, which is projected from the front reflection surface to an eye point of the occupant, and an optical axis of the image, which is projected from the back reflection surface to the eye point of the occupant, are overlapped with each other. Thereby, visual misalignment (double vision) of the virtual images respectively formed by the reflection surfaces can be limited to improve visual recognition of the optical display image.
However, in the HUD device of the Patent Literature 1, a radius of curvature of the back reflection surface, which is configured into a form of a curved convex surface, is smaller than a radius of curvature of the front reflection surface, which is configured into a form of a curved concave surface, so that due to a convex lens effect, an imaging point of an external image is placed closer in comparison to imaging points of the virtual images respectively formed by the reflection surfaces. As a result, when the occupant focuses on the positions of the virtual images respectively formed by the reflection surfaces, simultaneous visual recognition of the external image, which has the closer imaging point, becomes difficult, thereby being not desirable.    Patent Literature 1: JP2012-58688A